The Turkana Rift of East Africa is known for both a rich record of early human fossils and intense volcanic activity driven by shifting tectonic plates. Now, scientists report that the crust beneath the region is much thinner than previously thought, pointing to the long-term disintegration of the African continent and offering a new explanation for why so many ancient human remains were preserved there.
The findings were published in nature communication.
A giant crack formed by the movement of tectonic plates
The Turkana Rift stretches for about 500 kilometers across Kenya and Ethiopia and is part of the larger East African Rift System. This vast system extends from the Afar Depression in northeastern Ethiopia to Mozambique, separating the African tectonic plate from the Arabian and Somali plates. In the Turkana region, the African and Somali plates are slowly moving apart at a rate of about 4.7 millimeters per year.
As this separation occurs, a process called rifting pulls the crust sideways. The stress causes the surface to bend and crack, causing magma to rise upward from within the Earth.
Not all rifts completely divide continents. However, in this case, the Turkana Rift appears to be on the same path.
Scientists discover unexpectedly thin layer
The study’s lead author is Christian Rowan, Ph.D. Says, “We found that the rift in this area is more advanced, and the layer is thinner, than anyone had recognized.” student at Columbia University’s Lamont-Doherty Earth Observatory, part of the Columbia Climate School. “East Africa is further along in the transfer process than ever before.”
To reach this conclusion, Rowan and his colleagues analyzed a rare set of high-quality seismic data collected in collaboration with industry partners and the Turkana Basin Institute, founded by the late paleoanthropologist Richard Leakey. By investigating how sound waves travel through underground layers and combining those results with other imaging methods, the team mapped sediment structures and determined the depth of the layer beneath the crack.
Along the center of the rift, the crust is only 13 kilometers thick. Far away, it is more than 35 kilometers. This dramatic difference points to a process called “necking.”
“Necking” indicates an important tectonic stage
The term describes how the crust expands and thins in the middle, similar to the compressed “neck” that forms when a piece of saltwater taffy falls apart. As the crust becomes thinner, it also becomes weaker, making it easier to continue breaking.
“The thinner the crust gets, the weaker it becomes, which helps promote continued cracking,” says Rowan. Ultimately, the scab may break off completely.
“We have reached that critical threshold of crustal rupture,” says Anne Bessel, a geophysicist at Lamont and co-author of the study. “We think that’s why it’s more likely to fall apart.”
Yet, these changes occur over vast timescales. The Turkana Rift began opening about 45 million years ago, and researchers estimate that necking began after a massive volcanic eruption about 4 million years ago. The next phase, known as oceanization, may take several million more years to begin. At that stage, magma would rise through fractures to create new seafloor, and water from the Indian Ocean to the north could eventually flood.
Evidence of earlier failed rifting
The team also uncovered signs of an earlier rift episode that did not lead to complete continental separation. Instead, it caused the layer to become thinner and weaker, setting the stage for the current phase of activity.
“This challenges some of the more traditional ideas about how continents break apart,” says Rowan.
Since the Turkana Rift is the first known active continental rift currently undergoing necking, it provides scientists with a rare opportunity to study this important stage of tectonic evolution.
Co-author Folarin Kolawole, who is also with Lamont, says, “In short, we now have a front row seat to observe a critical rift phase that fundamentally shaped all rift margins around the world.” These processes are closely linked to other Earth systems, helping researchers reconstruct past landscapes, vegetation, and climate patterns. “We can then use that knowledge to understand what’s going to happen in our future, even on shorter time scales,” Bessel says.
Rethinking the fossil record of human evolution
The discoveries also shed new light on the region’s extraordinary fossil record. The Turkana Rift has produced more than 1,200 hominin fossils over the past 4 million years, about a third of all such discoveries in Africa. Many scientists have long viewed this region as a major center of human evolution.
Rowan and colleagues suggest another possibility.
Following extensive volcanic activity about 4 million years ago, land subsidence in the rift caused the beginning of necking. This subsidence created conditions where fine-grained sediments were rapidly deposited, which are ideal for preserving fossils.
“The conditions were right to preserve a continuous fossil record,” says Rowan.
This means that the Turkana Rift may not have been uniquely important as a place where human ancestors evolved, but rather a place where geological conditions made it easy to record their history.
This idea is just a hypothesis, but it opens up new avenues for research. “But other researchers can now use our results to explore those ideas,” says Rowan. “Furthermore, our results can be fed into tectonic models coupled with climate to really explore how changing tectonics and climate have affected our evolution.”
The research team also includes Paul Betka from Western Washington University and John Rowan from the University of Cambridge.